MY EXPERIENCES
I had installed database software with only "Create Software Only" option. Then i created database from Database Configuration Assistant. As i guessed, there occured some administrative problems because when i check other styles of installing from youtube,there was a password entry under "Administrative Password" with creating database option meanwhile installing.So do installation step by step by creating database option.Otherwise, there will be some configuration problems when you try to connect to database with 'sqlplus / as sysdba' command.
Then I installed database by creating database option to my other PC then the problem solved automatically.
There also might occur some problems about creating TNS AND LISTENER files. Because when you create database while installing software, it creates these files automatically.Once again,software must be installed with database creation option in my opinion.Even though i had solved this problem with some tricks as i mentioned below.
OTHER SOLUTIONS
Even though i had errors as i mentioned above. I solved it with another tricks. Here is the problem 
TRICK 1: You can connect with 'sys' username to database as 'sysdba' with conn sys / as sysdba command as shown here.
TRICK 2:
Connecting database with Local Naming Method ''sqlplus UserName@DatabaseName' then you will enter password;
Another style of Local Naming Method "sqlplus UserName/Password@DatabaseName" then it will connect database automatically;
